Title: In vitro and in vivo evaluation of cypermethrin, amitraz, and piperonyl butoxide mixtures for the control of resistant Rhipicephalus (Boophilus) microplus (Acari: Ixodidae) in the Mexican tropics

Interpretive Summary: Acaricide resistance is a major problem for the control of the southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), in many parts of the world. Resistance to major classes of acaricide, including pyrethroids and amitraz, in Mexican tick populations remains a major threat to the USDA's Cattle Fever Tick Eradication Program. In order to develop an effective resistance management strategy, scientists at the USDA, ARS, Knipling-Bushland Livestock Insects Research Laboratory in Texas conducted a joint study with Mexican researchers to evaluate the effectiveness of acaricide mixtures against resistant tick populations in Yucatan, Mexico. Both laboratory larval toxicity bioassays and on-animal efficacy trials were conducted to evaluate the effectiveness of various mixtures of cypermethrin, amitraz, and piperonyl butoxide (PBO) against resistant ticks. A significant synergistic effect was observed when the two acaricides and PBO were mixed in certain proportions in laboratory bioassays. The mixture acaricide formulation provided >95% control for over 28 days when tested against naturally infested ticks on-animal in a field study. The results from this study may facilitate the adoption of new synergized acaricide formulations for the control of resistant R. microplus ticks in Mexico and other Central and South American countries.

Technical Abstract:
The southern cattle fever tick, Rhipicephalus (Boophilus) microplus (Canestrini), is a haematophagous arachnid (Acari: Ixodidae) recognized globally as an economically important ectoparasite of cattle in tropical and subtropical agroecosystems. Populations of this invasive tick species around the world have developed resistance to all major classes of acaricides due to the sustained and indiscriminate use of acaricidal products. Resistance to synthetic pyrethroids is one of the most serious problems in tick control worldwide. Pesticide resistance management strategies like the use of pesticide mixtures or synergised pesticide formulations have shown to be effective in controlling resistant arthropods. The efficacy of a synergized pyrethroid to control natural infestations of pyrethroid-resistant R. microplus populations in the Mexican tropics is reported here for the first time. The toxicity of cypermetrhin, amitraz, and PBO alone or in mixtures was evaluated against R. microplus larvae in laboratory bioassays. Molecular assays were performed to determine if the acaricide mixture formulation helped eliminate resistant ticks showing the homozygous resistant genotype associated with target site insensitivity for pyrethroids. To corroborate the synergist effect found in vitro, an in vivo field study was conducted in Yucatan to control natural infestation of R. microplus ticks on cattle. The optimal concentration of the two acaricides and PBO to achieve persistent efficacy >95% for at least 28 days PT in the field was cypermethrin 0.2%+PBO 0.005%+amitraz 0.0125%. Although it is a potentially useful tool to combat pyrethroid resistance, a product based on an acaricide mixture like the one tested in this study has to be used rationally. Such a strategy needs to be part of an integrated tick control program that incorporates other technologies to minimize the economic impact of R. microplus and other ticks affecting cattle in a sustainable manner. Stewardship of acaricidal combination products is warranted as it has been demonstrated that some agriculturally damaging acari species can develop resistance to pesticide mixtures.